As the energy-saving and environmental issues have become increasingly prominent, lithium ion (Li-ion) battery, due to its advantages of high specific energy and green environmental protection, have been widely used as large capacity power supply in various fields such as electronic-powered automotive, aerospace, ship gradually. With the development of li-ion battery technology, energy density of li-ion battery becomes higher and higher, quantity of battery units in a battery pack also becomes larger and larger. After long-time use of a battery pack, asymmetry developed among the batteries in the battery pack can cause one or more of the batteries overcharging or over-discharging, and subsequently lowers the performance of the battery pack in the whole, resulting in serious effect on the service life of the battery pack. Therefore, a battery management system for managing and monitoring the working state of the battery pack is indispensable.
In practice, state of charge (SOC) is an important reference parameter of the working state of the li-ion battery pack, and is usually employed to indicate remainder energy of the li-ion battery pack. Accurate SOC estimation of the li-ion battery pack utilized in automobiles can not only tell drivers of correct estimated mileage of the automobiles, but also ensure charging/discharging optimization of the li-ion battery pack, which ensures safe utility of the li-ion battery pack. When the automobile is running, large currents may cause the battery pack to be overly discharged and subsequently destroy the battery pack. Therefore, real time collection of voltage, temperature, and charging/discharging current of each battery is important for accurate SOC estimation of the battery so as to prolong the life of the battery pack and increase performance of the automobile.
The SOC can be estimated based on attribute parameters such as voltage, current, resistance, temperature of the battery. The attribute parameters of the battery generally can change in accordance with the aging of the battery and other uncertain factors, such as random road conditions the automobile is going through.
Currently, the most popular method for SOC estimation of battery is the so-called ampere-hour method, which is also a relatively accurate method on SOC estimation. The ampere-hour method employs real time current integral to calculate ampere hour, and then revises temperature, self-discharging data and ageing parameters that can affect the SOC estimation, and eventually obtains a relatively accurate SOC value by using a revision function and said parameters. However, the above-mentioned method is still far from being sufficient for practical situations because there are many other factors that could practically affect SOC estimation of the battery, and because it is hard to achieve the revision function in practice. Therefore, to date the SOC value estimated by employing the ampere-hour method can be far from the real SOC value of the battery. Other existing methods for SOC estimation include constant current/voltage method, open circuit voltage method, specific density method, and so on. These methods each have more or less defects that would lead to inaccurate SOC value. Therefore, a novel method needs to be developed for accurate SOC estimation of a battery pack.